Thermostat sheath



Dec. 13, 1960 L. R. ERWIN 2,963,909

THERMOSTAT SHEATH Filed March 22, 1956 INVENTOR. LEE R. ERWIN.

BY W M ATTORNEY.

United States Patent THERMOSTAT SHEATH Lee R. Erwin, Duarte, Calif., assignor to Carrier Corporation, Syracuse, N.Y., a corporation of Delaware Filed Mar. 22, 1956, Ser. No. 573,257

1 Claim. (Cl. 73-343) This invention relates to a sheath for a thermostat and more particularly, to a sheath for a thermostat used in a hot water heater.

One of the major problems facing manufacturers of hot water heaters resides in corrosion. In recent years,

however, manufacturers have substantially prolonged the life of hot water heaters by lining the tanks of the heaters with glass and similar ceramic materials. In the main, this lining has greatly diminished corrosion in tanks due to oxidation. However, it has aggravated the problem of galvanic corrosion. Galvanic corrosion results from the use of dissimilar metals in hot water heater tanks. Copper components, for example, cause less noble metals to sacrifice themselves to copper through the medium of the electrolyte Which in the case of a hot water heater is the water in the tank.

In combating galvanic corrosion attempts have been made to protect components exposed to the water against such corrosion. It has been proposed to place within the tank an anode of a less noble metal which will sacrifice itself in preference to the tank components. It has also been proposed to insulate components subjected to galvanic action with suitable insulating materials.

Generally hot Water heaters contain a thermostat tube usually made of copper which causes ferrous boiler components to be subjected to galvanic corrosion. In actual practice this thermostatic element consists of a copper tube approximately 5" long which extends into the hot water tank and controls the application of heat to the water in the tank. It will be appreciated that in lining a tank with a glass or ceramic material there will always remain minute discontinuities of bare metal surface so that with the new tank lining practices the amount of copper surface exposed to the water in the tank greatly exceeds the amount of bare ferrous metal. Since the copper is the nobler metal in the electromotive series, the less noble ferrous metal sacrifices itself to the nobler metal through the medium of the electrolyte which as previously indicated is the water in the tank. This type of corrosion has been sufiiciently severe to cause failure of tanks of this nature within one year of service.

To prevent tank failure due to galvanic action two approaches have been utilized, both singularly and in combination. A common device is to use a magnesium anode within the water tank. The magnesium being of a less noble nature than the iron and copper will sacrifice itself to the copper in preference to the iron because of its higher position in the electromo-tive series. It has been found, however, that upon installation and repair, plumbers very often remove these anodes without replacing them and thereby cause eventual tank failure.

Another approach to minimize galvanic corrosion resides in the isolation of the copper thermostat tube. Plastic coatings have been applied to these tubes; how ever, they subsequently fail due to osmosis and break down rapidly. Another difliculty involved in the use of plastic coatings resides in their insulating properties which 2,963,909 Patented Dec. 13, 1960 cause a heat transfer lag which upsets the thermostat differential.

Another approach has been to tin plate the thermostat tube. Tin has been used because it is near iron in the electromotive series. This approach does not solve the problem but rather minimizes the amount of current flow due to galvanic action and in effect prolongs the life of the tank by decreasing the rate of galvanic corrosion.

The application of a glass coating directly to the thermostat tube has been suggested. In practice great difiiculties are incurred due to the complete annealing of the thermostat tube. This tube becomes very soft after firing the glass coating and the tube bends while being assembled. Similarly it has also been suggested that a glass tube be attached to the thermostat tube by means of a suitable adhaive such as a lead base adhesive which would assist heat transfer. In practice this approach has also been unsuccessful due to the widening of the thermostat differential and the added hazard created by possible breakage of the glass sheath during assembly.

The chief object of this invention is to provide a thermostat tube for use in hot Water heaters which will be shielded from galvanic corrosion.

Another object of the invention is to provide a glass coated tubular copper shield for a hot Water heater thermostat tube which will alleviate or eliminate galvanic corrosion of the boiler components. Other objects will be readily perceived from the following description.

This invention relates to a shield for a thermostat tube which projects into the tank of a hot water heater. The shield or sheath comprises a copper tube which is in substantial surface contact with the thermostat tube. This shield is provided with a glass coating which forms in effect an insulating blanket which prevents galvanic corrosion of the heater components.

This invention also discloses a method for applying a, glass coating to a copper tube for use as a shield for a thermostat inserted in a hot Water heater tank in which the step consist in closing one end of the copper tube, dipping the copper tube in a vertical position with its open end downward into an enamel slip, compressing the air within the tube sufficiently to coat a substantial area within the tube with slip, removing the tube from the slip, drying the tube and baking the tube in a temperature of substantially 1600 F. for approximately seven to ten minutes.

The attached drawings illustrate a preferred embodiment of the invention, in which Figure l is a view partially in section and partially in elevation illustrating the invention utilized in a hot water heater;

Figure 2 is a fragmentary sectional view of the invention; and

Figures 3, 4 and 5 are views illustrating a method of applying the glass coating to the tube.

Referring to the drawings, there is shown in Figure 1 a water heater 2 which utilizes the present invention. This water heater comprises an outer casing 3 and an inner cylindrical wall 4. Insulation 5 is inserted between the casing 3 and the inner wall 4. This structure is mounted upon a base 6. Mounted within the base and in the lower portion of the inner wall 4 is gas burner 7. Any other suitable combustible or electrical source of heat may be utilized. In the present embodiment the gas burner is of a conventional type in which the gas supply induces primary air into a venturi. The air and gas mix and pass into a plenum from which the mixture is emitted through suitable openings, mixed with secondary air and then burned. Adjacent the openingsdn the burner is a pilot light 8. Both the pilot and the gas burner are operatively associated with a control 11 by means of gas lines 9 and 10. Control 11 has included 3 therein a protuberance 12 which is a thermostatic sensing element the function and operation of which will be described hereinafter.

Extending through the casing and the inner wall is an opening in which is inserted a suitable drain valve 14 which is conventionally found on heaters of this nature. A water inlet tube 15 and an anode 16 extend into the tank. This anode may be of magnesium or any other suitable element in the electromot-ive series which is substantially higher than iron.

concentrically extending through the tank 13 is a flue 17 which has therein a suitable fiue bafile (not shown). Separating the tank 13 from the combustion chamber wherein the burner 7 is located is a suitable partition 18. In the present embodiment this partition has a bell shape. In the upper portion of the water heater are located suitable connections 19 and 2% to the tank. Connected to the flue 17 is a suitable draft diverter 21.

A conventional water heater arrangement has been described. The structure comprises an insulated tank having a combustion chamber in a lower portion thereof. Above said combustion chamber is a tank filled with Water which is to be heated. The combustion chamber has located therein a suitable burner or other source of heat which heats the water in the tank. This burner is attached to a control which has a sensing element located within the tank filled with water. As the water temperature decreases this control permits a suitable amount of fuel to reach the burner and thereby increase the temperature of the water. Water is induced into the tank through the tube 15 which introduces the water at the lower extremity of the tank. As he water is heated it rises to the top of the tank and may be removed through a suitable opening as for example connection 19. Extending parallel to this tube 15 is a magnesium anode 16 which as previously set forth participates in a galvanic reaction with the thermostatic element 12 for the purpose of shielding ferrous components from similar galvanic corrosion. The heat exchange structure of the present water heater is of a conventional type in which heat is transferred directly through the partition 18 and through the fine 17. Located in the flue may be suitable baflies which will increase the amount of heat transfer between flue gases and the water in the tank.

Referring to Figure 2 there is indicated an enlarged view of the thermostat arrangement. The thermostat 12 comprises a copper cylindrical element 3t? with an open end 32 and having a suitable metal rod therein such as Invar metal. As the element is heated the difference of the coefficients of expansion of the two metals is utilized to actuate the control 11 which in turn controls the flow of fuel to the burner 7. T his thermostatic element is enclosed by an open sleeve 32. This sleeve is made of copper and is in substantial surface contact with the thermostat tube 3%. This sleeve 31 is covered with a suitable ceramic such as glass and forms a coating extending substantially over the entire outer surface of the sleeve and partially extends over the inner surface of the sleeve 31 which is not in surface contact with the thermostat tube 30. The sleeve 31 with the enclosed thermostat tube 30 is held into place by a suitable bushing 33 which is connected to a threaded connection welded to the inner cylindrical wall 4 of the hot water heater. Covering the opening through the casing 3 is a suitable shield 35.

Viewing the thermostat tube as disclosed with its glass coated shield it is seen that the thermostat tube is completely enclosed by the sleeve 31, and is in substantial surface contact therewith. The glass coating extends into the inside of the tube substantially to the point where the thermostat tube terminates therein.

In openation, the galvanic phenomenon generates a current when the thermostat is protected by the glass coated sheath of approximately the current generated in a hot water heater utilizing a bare unprotected copper thermostat. It has been found that maintaining the sleeve end open does not noticeably increase the amount of current generated through galvanic action. Permitting the sleeve to remain open allows smaller clearances between the thermostat tube and the sleeve and obviates the problem of removing air entrapped in the closed end of the tube upon assembling the thermostat tube within its protective sheath.

Referring to Figures 3, 4 and 5 I have schematically indicated the manner in which the coating is applied comprising the following steps: First one end of the sleeve is closed by a cork 40 or similar device having carrying means 45. The coating is applied by dipping the sleeve 31 in a vertical position into a slurry or slip 46 comprising parts glass frit; 5-7 parts enamelers clay; to A; parts sodium nitrite; A to /8 parts bentonite; A to 1 parts borax; to A3 parts magnesium carbonate; /8 to parts urea; 40 to 45 parts water. This enamel slip is obtained by grinding the mixture in ball mills and the resulting slurry or slip is controlled to obtain the desired amount of pickup.

The sleeve is dipped into the slurry and the hydrostatic head of the slurry compresses the air within the tube so that a length 47 of approximately within the interior of the sleeve is coated with the slip. After dipping, the tube is dried and placed in a suitable furnace 48 such as indicated in Figure 5. Within said furnace 48 is located a suitable source of heat 49 which is capable of firing said furnace to a temperature of approximately 1600 F. The tube is kept within the furnace approximately seven to ten minutes depending upon the heat recovery of the furnace. After firing, the parts are allowed to cool and they are inspected and passed on to assembly.

During assembly the thermostat tube is treated at one end with a suitable adhesive, for example, a silicone adhesive such as Dow Corning 40C. The coated sleeve is then slipped over the tube and bonded thereto by the adhesive.

This invention relates to a unique sheath for a thermostat tube as utilized in a hot water heater which is substantially impervious to galvanic reaction with the ferrous portions of the hot water heater. Utilizing the present invention the thermostatic differential is not noticeably impeded by the sleeve and therefore the thermostat will sense changes of temperature in an efiicient manner unlike the plastic coatings previously suggested. By coating the sheath the life of the Water tank is not only increased by substantially terminating galvanic action but also the life of the magnesium anode is also prolonged thereby imparting to the hot water heater the combined protective approaches of isolating reactive elements and utilizing sacrificing elements.

There is also presented a unique method for applying a suitable ceramic coating to the sheath which in a single operation coats both the inner and outer surfaces of the tube which are normally exposed to galvanic action without interposing a similar glass coating between the sheath and the thermostat element which might impair the thermostatic differential therebetween.

While I have described preferred embodiments of the invention, it will be understood that the invention is not limited thereto since it may be otherwise embodied within the scope of the following claim.

I claim:

In a thermostat assembly for a hot water heater, the combination of a cylindrical copper thermostat tube, a copper sleeve having an open end enclosing said thermostat tube and being in substantial surface contact therewith, said sleeve having a ceramic coating which covers the outer surface of the sleeve and References Cited in the file of this patent UNITED STATES PATENTS 5 664,764 Koehring Dec. 25, 1900 794,012 Hepler July 4, 1905 1,337,529 Otto Apr. 20, 1920 1,520,549 Otto et a l. Dec. 23, 1924 10 6* Humphrey Apr. 20, 1926 Johnson Mar. 21, 1933 Smith July 18, 1933 Foote Mar. 26, 1935 Sweo Oct. 24, 1939 Raymond et a1. Nov. 17, 1942 Vollrath July 30, 1946 Bennett July 5, 1949 

